Risk Assessment of “Other Substances” - L-cysteine and L-cystine

ABSTRACT

The Norwegian Scientific Committee for Food Safety (Vitenskapskomiteen for mattrygghet, VKM) has, at the request of the Norwegian Food Safety Authority (Mattilsynet; NFSA), assessed the risk of “other substances” in food supplements and energy drinks sold in Norway. VKM has assessed the risk of doses given by NFSA. These risk assessments will provide NFSA with the scientific basis while regulating the addition of “other substances” to food supplements. “Other substances” are described in the food supplement directive 2002/46/EC as substances other than vitamins or minerals that have a nutritional or physiological effect. It is added mainly to food supplements, but also to energy drinks and other foods. VKM has not in this series of risk assessments of “other substances” evaluated any claimed beneficial effects from these substances, only possible adverse effects. The present report is a risk assessment of L-cysteine and L-cystine, and is based on previous risk assessments of these amino acids and articles retrieved from a comprehensive literature search. In this report L-cysteine and L-cystine are often termed merely cysteine and cystine, respectively. L-cysteine is a central compound in sulphur metabolism in the human body. L-cysteine is a conditionally essential sulphur-containing amino acid, obtained from L-methionine and from serine. Sulphur-containing amino acids are mainly found in cereal proteins and animal proteins, and less abundantly in pulses. Cysteine may occur in proteins either as cysteine itself or as cystine. Cystine is the disulphide dimer of cysteine, and is a more stable compound than cysteine. According to information from the Norwegian Food Safety Authority (NFSA), cysteine and cystine are ingredients in food supplements purchased in Norway and NFSA has requested a risk assessment of the following doses of cysteine and cystine in food supplements L-cysteine 10 mg/day and L-cystine 250, 500, 750 and 1000 mg/day. The mean usual daily intake of cysteine in the USA for all life stage- and gender groups is 1.0 g/day (NHANES II, USA). Because there are few intervention studies with cysteine or cystine, studies with N-acetylcysteine (or N-acetyl-L-cysteine, NAC), which is readily converted to cysteine, is included in this risk assessment. NAC is used as a pharmaceutical drug for various conditions, mainly as mucolytic agent, as paracetamol antidote, and has been included in numerous clinical trials. Most of the cited studies have tested NAC in doses of about 600-1200 mg/day. The study groups have been various patient groups which included children, adolescents, adults and elderly, however relatively few studies have been conducted in children. In the randomised controlled trials there have been no differences in severe adverse events between the placebo and NAC-groups. The adverse effects reported are generally limited to mild gastrointestinal symptoms. The dose 1200 mg of NAC yields maximum 900 mg of L-cysteine or L-cystine. In adults, it is well documented that doses up to 900 mg per day for one year (corresponding to 13 mg/kg bw/day in a 70 kg adult) is without appreciable health risk. The data for doses above 900 mg/day are more scarce. There are no data indicating that children and adolescent are more vulnerable than adults for L-cysteine or L-cystine. No tolerance level is set for cysteine or cystine specifically for children or adolescents, but an assumption is made that these age groups have similar tolerance per kg body weight as adults. VKM concludes that In adults (≥18 years), the specified doses 10 mg/day L-cysteine and 250, 500 and 750 mg/day L-cystine in food supplements are considered to be unlikely to cause adverse health effects, whereas the dose 1000 mg L-cystine per day may represent a risk of adverse health effects. In adolescents (14 to <18 years), the specified doses 10 mg/day L-cysteine and 250, 500 and 750 mg/day L-cystine in food supplements are considered to be unlikely to cause adverse health effects in adolescents, whereas the dose 1000 mg L-cystine per day may represent a risk of adverse health effects. In children (10 to <14 years), the specified doses 10 mg/day L-cysteine and 250 and 500 mg/day L-cystine in food supplements are considered to be unlikely to cause adverse health effects, whereas the doses 750 and 1000 mg L-cystine per day may represent a risk of adverse health effects. Children below 10 years were not included in the terms of reference.